Optimisation of Stilling Basin for an Ogee Spillway for Enhanced Energy Dissipation

Spillways are meant for disposing of the excess flood safely to the downstream without causing any damage to its upstream, the structure itself and its downstream. Energy dissipator is essential component of spillway which dissipates the incoming excess energy. The most common energy dissipators are ski jump buckets, roller buckets, hydraulic jump type stilling basins, etc. Among these, stilling basins are very popular type of energy dissipators provided for high head / low head spillways, spillways, culverts and channels. Energy dissipation by stilling basins is governed by various factors like intensity of discharge, head causing flow, Froude number and tail water depth. When the tail water levels are sufficient to cope up with the sequent depth of hydraulic jump, stilling basins with horizontal apron are provided. If the tail water levels are higher than the required for sequent depth, sloping aprons are provided to contain the jump within the spillway glacis to avoid encroachment of jump further upstream. The design of horizontal basin involves fixing of length of basin, appurtenant structures like chute blocks, baffle blocks and end sill, whereas design of sloping apron involves fixing of slope of apron in addition to other components for horizontal apron. The slope of the apron must be determined from economic considerations and the length must be judged by the type and soundness of the riverbed downstream. The end sill is constructed at the downstream end of the stilling basin, whether solid or dentate and has function of reducing the length of the hydraulic jump and controlling scour. The performance of spillway depends upon the efficiency of its energy dissipator. In this paper, three different layouts of stilling basins, namely horizontal, sloping and curved aprons were studied, to achieve maximum energy dissipation. The study was carried out using both physical and numerical modelling techniques. Numerical modelling was done by using FLOW 3D HYDRO software. From the simulations, it can be concluded that a curved stilling basin would provide more energy dissipation than sloping or horizontal layouts.